Airbag systems are commonly used in automotive applications to provide protection for the vehicle operator and/or passenger in the event of a vehicular collision. A popular technique for implementing an airbag system includes detecting vehicular acceleration via an accelerometer and then evaluating the resulting acceleration signal to determine whether an impact of sufficient severity has occurred to require the airbag to deploy.
Most acceleration-based airbag systems utilize a microprocessor to evaluate the acceleration signal. As is known in the art, such microprocessor use permits evaluation algorithms to be easily implemented in software. In so doing, the input analog acceleration signal is converted to a digital word, and all subsequent signal processing by the microprocessor is implemented digitally.
An example of one known microprocessor-based system for evaluating an acceleration signal is shown in FIG. 1 as system 10. Referring to FIG. 1, system 10 includes an accelerometer 12 which may be a micro-machined piezoresistive sensor whose differential analog output voltage (S+-S-=V.sub.IN) is proportional to the applied acceleration. The differential acceleration signals S+ and S- are applied to signal conditioning circuitry 14 via signal lines 16 and 18, respectively. The signal conditioning circuitry typically includes a gain stage and temperature compensation circuitry, and provides a conditioned acceleration signal V.sub.OUT at output 20. A microprocessor 24 is provided to evaluate V.sub.OUT and includes an input 22 connected to output 20 via signal path 26. Microprocessor 24 receives the conditioned analog acceleration signal V.sub.OUT, converts the analog signal to a digital signal, and implements a software algorithm to evaluate the digital signal and determine whether the airbag should be deployed. Microprocessor 24 then controls airbag deployment circuitry via signal path 28.
Software algorithms for evaluating acceleration signals to determine airbag deployment may be implemented in a number of ways. A conventional approach is to use a time-dependent algorithm wherein the algorithm begins when a predefined level of acceleration is exceeded. With the time-dependent approach, the digital acceleration signal is digitally integrated within microprocessor 24, and the resulting predetermined velocity curve is evaluated against a predetermined curve to determine if a deploy event has occurred. Referring to FIG. 2, this approach is shown graphically. FIG. 2 shows a plot of velocity versus time wherein curve 30 represents the maximum velocity allowed before a deploy is required, and curve 40 represents a velocity below which a deploy event should not occur and below which system 10 is reset. The break points and relative slopes of curves 30 and 40 can be adjusted by software to optimize system 10 for various vehicular applications. Additional breakpoints and slopes can be added, so long as there is sufficient memory in microprocessor 24 to store such data.
The foregoing microprocessor-based system and implementation thereof has a number of drawbacks. First, system 10 is designed around a process optimized for digital circuits, which requirements are inconsistent with the requirements for processing of analog signals such as those provided by analog accelerometer 12. Second, microprocessors are typically large and complicated integrated circuits, resulting in significant cost and area penalties for the circuit and system designers. Third, variations in the accuracy of accelerometer 12, along with the finite resolution of the data converter of microprocessor 24, requires curves 30 and 40 of FIG. 2 to be some minimum distance apart. This limits the accuracy of the algorithm and may delay a deployment of the airbag beyond the time when deployment should actually occur. Moreover, the finite resolution of the data converter of microprocessor 24 introduces error into the algorithm which can be cumulative, and in some cases unacceptable. Fourth, most low cost microprocessors process data at a relatively slow rate. This limits the number of break points and slopes which can be used in a time-dependent algorithm, which may then result in missing important information which occurs at too rapid a rate for the digital system to handle (known in the art as aliasing).
To avoid the foregoing drawbacks of a microprocessor-based acceleration signal evaluating system, it is desirable to implement an analog signal processing system for evaluating the analog acceleration signal. However, implementation of a time-dependent analog algorithm is a difficult task and very area intensive in the design of integrated circuitry to accomplish such an algorithm. What is therefore needed is an analog signal processing system implementing a time-independent algorithm to thereby eliminate or minimize the resolution constraints associated with data converters, and significantly reduce timing uncertainty and aliasing problems. An added benefit of implementing a time-independent algorithm is that it avoids secondary events, such as hitting a curb, for example, having any effect on the analog deployment algorithm.